Opegrapha physciaria
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| Opegrapha physciaria | |
|---|---|
 | |
| Asci and ascopores of Opegrapha physciaria, 1000x magnification. The fungus was growing on the foliose lichen Xanthoria parietina. | |
Scientific classification  | |
| Kingdom: | Fungi |
| Division: | Ascomycota |
| Class: | Arthoniomycetes |
| Order: | Arthoniales |
| Family: | Opegraphaceae |
| Genus: | Opegrapha |
| Species: | O. physciaria |
| Binomial name | |
| Opegrapha physciaria | |
| Synonyms[1] | |
Opegrapha physciaria is a species of lichenicolous (lichen-dwelling) fungus in the family Opegraphaceae.[2] This tiny black fungus lives harmlessly within the tissues of the common sunburst lichen (Xanthoria parietina), producing small dark disc-shaped fruiting bodies that emerge from the host's surface. It is found in Europe and North America, typically growing on tree bark in woodlands where its host lichen forms bright orange patches.
It was first formally described as a new species in 1897 by the Finnish lichenologist William Nylander, who placed it in the genus Lecidea.[3] David Hawksworth and Brian J. Coppins transferred it to the genus Opegrapha in 1992.[4]
Description
Opegrapha physciaria is a minute fungus that lives parasymbiotically on the common sunburst lichen (Xanthoria parietina). It threads its colourless hyphae through the host's tissue rather than forming a thallus of its own; the only part visible to the naked eye is its reproductive bodies. These begin life submerged in the lichen, then burst through the surface as tiny, black, non-powdery discs (ascomata) about 0.7 mm across. When first exposed the discs are roughly circular, but they may elongate slightly with age; several often crowd together in dense patches, while others remain solitary. A firm, continuous wall (excipulum) surrounds each disc and merges below them, measuring roughly 40–45 μm in height. Inside, a clear hymenium about 65 μm tall is threaded with branched, gel-coated filaments (paraphysoids) whose swollen brown tips form a pseudo-epithecium that roofs the spore layer. Treatment with iodine after a mild alkali wash turns the hymenium blue, indicating the presence of starch-like compounds.[5]
The asci are bitunicate—built from two separable walls that pop apart to discharge the spores—and are club-shaped, 45–50 × 18 μm, each producing eight spores. Unlike many relatives they lack the dark, starch-positive ring usually seen at the ascus apex and instead possess a small, pale ocular chamber through which the spores escape. The ascospores themselves are smooth, ellipsoid and three-celled (containing two internal cross-walls), measuring 12–16 × 6–8 μm. They start out hyaline (colourless) but darken to a warm brown as minute pigment granules accumulate on the outer coat (perispore), which is only about 1 μm thick. Nearby, sunken black pycnidia release curved, colourless conidia 6–7.5 × 1 μm that may assist in asexual spread. Together these features—immersed, black, erumpent discs; three-celled spores that brown with age; and immersion in Xanthoria tissue without damaging it—distinguish O. physciaria from similar lichen-dwelling species.[5]